Oxide high-critical temperature superconductor acicular crystal and its production method

ABSTRACT

The present invention relates to a defect-free oxide high-critical temperature superconductor acicular crystal, that is, an oxide high-critical temperature superconductor acicular crystal that is substantially a perfect crystal and also relates to a method for producing the same, wherein such a crystal is essential for achieving superconducting electronic devices. The oxide high-critical temperature superconductor acicular crystal of the present invention includes an acicular crystal having a Bi 2 Sr 2 Ca 2 Cu 3 O 10  (Bi-2223) crystal structure and is grown from a powder compact by heat-treating the powder compact in an oxygen atmosphere, wherein the powder compact contains an oxide having the Bi-2223 crystal structure and TeO 2 , CaO, or (SrCa) 3 TeO 6 . The achievement of the acicular crystal having the Bi-2223 crystal structure contributes to the development of superconducting electronic devices that have been theoretically proposed but have not been achieved.

TECHNICAL FIELD

The present invention relates to a defect-free single crystal of an oxide high-critical temperature superconductor which is essential for obtaining superconducting electronic devices, that is, an oxide high-critical temperature superconductor acicular crystal which is substantially a perfect crystal and also relates to a method for producing the same.

BACKGROUND ART

A single crystal of an oxide high-critical temperature superconductor has a crystal structure in which conductive layers and non-conductive layers are alternately stacked and the layers form an intrinsic Josephson junction. In recent years, single-crystal switching element devices using the intrinsic Josephson effect have been proposed. The single-crystal switching element devices, which are of a new type, are one-hundredth the size of known Josephson junction devices and have a switching speed that is 100 times higher than that of the known devices. The new devices are presumed to have a high operating frequency of several THz (terahertz).

In a submicron crystal element including a Bi₂Sr₂Ca₂Cu₃O₁₀ acicular crystal, superconducting single electron tunneling effect that a pair of electrons passes in the crystal has been currently observed. In order to achieve this effect, the element must operate at liquid helium temperature (4.2 K). When crystals in which the number of layers in a unit cell is about 1000 are used, it is presumed that superconductive single electron pair elements operating at liquid nitrogen temperature (77 K) can be achieved.

In order to achieve these elements, crystals having no defect or few defect must be used. At the present, an acicular crystal of a Bi oxide superconductor has the highest performance. The oxide superconductor has two types of crystal structure: a Bi₂Sr₂Ca₁Cu₂O₈ (Bi-2212) crystal structure having a superconducting critical temperature of about 85 K and a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) crystal structure having a superconducting critical temperature of about 110 K. In research and development, acicular crystals having the Bi-2212 crystal structure, of which the growth can be achieved, have been used. The inventors have succeeded in growing the acicular crystals having the Bi-2212 crystal structure and extremely high crystallinity using a powder compact without performing a quenching operation and amorphizing operation, wherein the powder compact has nominal composition of growing the Bi-2212 acicular crystals and contains an element for reducing the melting point. The inventors have filed a patent application for the technique (Japanese Patent Application No. 2001-38170).

For the acicular crystals, the Bi-2223 crystal structure having a superconducting critical temperature of about 110 K, which is much higher than a liquid nitrogen temperature of 77 K, is extremely advantageous in view of practical use as compared with the Bi-2212 crystal structure having a superconducting critical temperature of about 85 K. However, the acicular crystals that have been obtained only have the Bi-2212 crystal structure and the growth of acicular crystals having the Bi-2223 crystal structure have not succeeded.

DISCLOSURE OF INVENTION

As described above, a method for producing an oxide high-critical temperature superconductor acicular crystal having no defects and the Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) crystal structure has not been established. The preparation of high-performance acicular crystals has not been also achieved.

Thus, it is a task to prepare the way for commercialization of superconducting electronic devices by developing a method for producing an oxide high-critical temperature superconductor acicular crystal having no defects and the Bi-2223 crystal structure and then preparing high-performance acicular crystals. Such devices have been theoretically proposed but have not been brought into practical use.

In view of the above situation, it is an object of the present invention to provide an oxide high-critical temperature superconductor acicular crystal having few defects and the Bi-2223 crystal structure, wherein such a crystal is essential for obtaining superconducting electronic devices. It is another object of the present invention to provide a method for producing such a crystal.

In order to achieve the above objects, the present invention provides the crystals and methods below.

-   (1) An oxide high-critical temperature superconductor acicular     crystal includes an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀     crystal structure, wherein the acicular crystal is grown from a     powder compact by heat-treating the powder compact at a temperature     of 840 to 890° C. in an atmosphere containing 5 to 100% of oxygen,     and the powder compact contains one mol of an oxide having the     Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of TeO₂. -   (2) A method for producing an oxide high-critical temperature     superconductor acicular crystal includes a step of heat-treating a     powder compact at 840 to 890° C. in an atmosphere containing 5 to     100% of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀     crystal structure, wherein the powder compact contains one mol of an     oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8     mol of TeO₂. -   (3) An oxide high-critical temperature superconductor acicular     crystal includes an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀     crystal structure, wherein the acicular crystal is grown from a     powder compact by heat-treating the powder compact at a temperature     of 840 to 890° C. in an atmosphere containing 5 to 100% of oxygen,     and the powder compact contains one mol of an oxide having the     Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, 0.2 to 0.8 mol of TeO₂, and 0.1     to 2.0 mol of CaO. -   (4) A method for producing an oxide high-critical temperature     superconductor acicular crystal includes a step of heat-treating a     powder compact at 840 to 890° C. in an atmosphere containing 5 to     100% of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀     crystal structure, wherein the powder compact contains one mol of an     oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, 0.2 to 0.8 mol     of TeO₂, and 0.1 to 2.0 mol of CaO. -   (5) An oxide high-critical temperature superconductor acicular     crystal includes an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀     crystal structure, wherein the acicular crystal is grown from a     powder compact by heat-treating the powder compact at a temperature     of 840 to 890° C. in an atmosphere containing 5 to 100% of oxygen,     and the powder compact contains one mol of an oxide having the     Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of an oxide     having a (SrCa)₃TeO₆ crystal structure. -   (6) A method for producing an oxide high-critical temperature     superconductor acicular crystal includes a step of heat-treating a     powder compact at 840 to 890° C. in an atmosphere containing 5 to     100% of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀     crystal structure, wherein the powder compact contains one mol of an     oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8     mol of an oxide having a (SrCa)₃TeO₆ crystal structure.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the preparation of an acicular crystal has succeeded according to the following procedure: a powdery oxide high-critical temperature superconductor having a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi-2223) crystal structure is mixed with a powder containing TeO₂, CaO, and the like; the mixture is formed into powder compacts; and the powder compacts are then heat-treated in an atmosphere in which the oxygen partial pressure is varied, thereby directly preparing the acicular crystal having the Bi-2223 crystal structure using the compact.

Known acicular crystals have been grown from a calcined multiphase powder. Therefore, it has not been able to grow the acicular crystal having the Bi-2223 crystal structure.

In the present invention, the acicular crystal having the Bi-2223 crystal structure is directly grown from the compacts prepared according to the following procedure: a single phase powder having the Bi-2223 crystal structure is prepared in advance by a special method; the single phase powder is mixed with powdery TeO₂, powdery TeO₂ and CaO, or powdery (SrCa)₃TeO₆, which enable the acicular crystal to grow; and the mixture is formed into the powder compacts. This production method and the acicular crystal grown thereby are completely new, and the acicular crystal having a superconducting critical temperature of 110 K can be obtained.

Embodiments of the present invention will now be described in detail.

(1) Effect of Single Phase Bi-2223 Crystal Structure

Investigation has been conducted on the growth of acicular crystals having a Bi-2201 crystal structure, Bi-2212 crystal structure, or Bi-2223 crystal structure, wherein the Bi-2201 crystal structure, Bi-2212 crystal structure, and Bi-2223 crystal structure have a superconducting critical temperature of 20 K or less, about 85 K, and about 110 K, respectively. During that process, the inventors found that the crystal structure of the acicular crystals depends on the crystal structure of powder compacts, namely, host phases. In order to obtain a defect-free, single-phase acicular crystal having the Bi-2223 crystal structure, the acicular crystal must be grown from a single-phase powder compact, namely, a host phase, having the Bi-2223 crystal structure.

(2) Effects of Addition of TeO₂ and Addition of Both TeO₂ and CaO

The growth of acicular crystals is promoted in proportion to a difference between the melting point of an oxide high-critical temperature superconductor and that of a host phase having nominal composition. Therefore, it is extremely effective that the composition contains TeO₂, which reduces the melting point of the host phase. An acicular crystal having the Bi-2223 crystal structure is grown from a mixture containing one mol of the oxide Bi₂Sr₂Ca₂Cu₃O₁₀ and 0.2–0.8 mol of TeO₂. When the TeO₂ content is about 0.5 mol, the optimum effect can be achieved. The obtained acicular crystal does not contain Te.

When the host phase contains both TeO₂ and CaO, the host phase has low melting point, thereby promoting the growth of the acicular crystal. When the host phase contains TeO₂ only or both TeO₂ and CaO, the host phase has a (SrCa)₃TeO₆ crystal structure.

(3) Effect of Addition of (SrCa)₃TeO₆

In order to grow the acicular crystal having the Bi-2223 crystal structure, the diffusional driving force for growing the acicular crystal in the host phase is necessary. (SrCa)₃TeO₆ contained in the host phase has the driving force. The acicular crystal is grown in a powder compact containing one mol of an oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2–0.8 mol of an oxide having a (SrCa)₃TeO₆ crystal structure. When the (SrCa)₃TeO₆ content is about 0.5 mol, the optimum effect can be achieved.

(4) Effect of Temperature and Atmosphere of Heat Treatment

In order to grow acicular crystals, the temperature and atmosphere of heat treatment must be optimized. The acicular crystal having the Bi-2223 crystal structure is grown at 840–890° C. in an atmosphere containing 5–100% of oxygen during the heat treatment. The optimum conditions are as follows: the heat-treating temperature is 860° C. and the oxygen content in the atmosphere is 10%.

EXAMPLES

(1) Effect of Single Phase Bi-2223 Crystal Structure

A powder with nominal composition for Bi-2223 was prepared by a chemical coprecipitation method. A powder compact prepared using the powder was heat-treated at 845–850° C. for 100 hours in an atmosphere containing 20% of O₂, thereby obtaining a single phase pellet having a Bi-2223 crystal structure. The pellet was crashed in anhydrous alcohol with a ball mill so as to avoid the hydrolysis of the pellet, thereby preparing a single phase powder having the Bi-2223 crystal structure. In order to render the Bi-2223 crystal structure to be a single phase, the powder must have nominal composition represented by (BiPb)₂Sr₂Ca₂Cu₃O₁₀ in which part of Bi is replaced with Pb.

As is generally known, the nominal composition is represented by Bi_(1.6-1.8)Pb_(0.3-0.4)Sr_(1.9)Ca_(2.1)Cu_(3.0)O_(x) in particular

On the other hand, a calcined multiphase powder with substantially the same nominal composition as the above was prepared, wherein the powder had the Bi-2212 crystal structure containing Ca₂CuO₃, Ca₂PbO₄, and the like. These powders were mixed with TeO₂ and CaO in such a manner that the ratio of an oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure to TeO₂ to CaO is 1:0.5:1.0 on a mole basis, the mixtures were calcined at 820° C. for 10 hours, and the resulting mixtures were then formed into powder compacts having a diameter φ of 15 mm and a thickness of 2 mm.

The powder compacts were heat-treated at 860° C. for 100 hours in an atmosphere containing 10% of oxygen, thereby growing acicular crystals from the respective powder compacts. Table 1 shows the crystal structure of the powder compacts, namely, host phases, and the crystal structure of the acicular crystals.

TABLE 1 Crystal Structure of Crystal Structure of Host Phase Acicular Crystal Calcined Powder Having Bi-2212 + Ca₂CuO₃ + Bi-2212 Nominal Composition for Ca₂PbO₄, and the like Forming Bi-2223 Crystal (TeO₂ + CaO) Structure Powder Having Bi-2223 Bi-2223 (TeO₂ + CaO) Bi-2223 Crystal Structure

The acicular crystal having the Bi-2212 crystal structure is grown from the host phase having the Bi-2212 crystal structure, and the acicular crystal having the Bi-2223 crystal structure is grown from the host phase having the Bi-2223 crystal structure. That is, the crystal structure of an acicular crystal depends on the crystal structure of a superconductor in a host phase. The single-phase acicular crystal having the Bi-2223 crystal structure can be grown only from the powder compact in which the host phase has the Bi-2223 crystal structure.

(2) Effects of Addition of TeO₂ and Addition of Both TeO₂ and CaO

An oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure was mixed with TeO₂ or both TeO₂ and CaO, thereby preparing mixed powders having different compositions. The powders were calcined at 820° C. for 10 hours and then formed into powder compacts having a diameter φ of 15 mm and a thickness of 2 mm. The powder compacts were heat-treated at 860° C. for 100 hours in an atmosphere containing 10% of oxygen, thereby growing acicular crystals from the respective powder compacts. Table 2 shows the length of the acicular crystals, obtained by varying the content of TeO₂ or the content of both TeO₂ and CaO, having the Bi-2223 crystal structure.

TABLE 2 Length of Acicular TeO₂ (mol) CaO (mol) Crystal (mm) 0 0 0 0.2 0 1~2 0.5 0 6~8 0.8 0 1~3 0.5 0.1 6~9 0.5 0.5 7~10 0.5 1.0 9~12 0.5 1.5 8~10 0.5 2.0 3~5

The acicular crystals having the Bi-2223 crystal structure are grown from the powder compacts containing one mol of the oxide Bi₂Sr₂Ca₂Cu₃O₁₀ and 0.2–0.8 mol of TeO₂. When the TeO₂ content is about 0.5 mol, the optimum effect can be achieved and the crystal length is 6–8 mm. For a combination of TeO₂ and CaO, the acicular crystals are grown from the powder compacts having a CaO content of 0.1–2.0 mol when the TeO₂ content is 0.5 mol which is the most effective value. When the CaO content is about 1.0 mol, the optimum effect can be achieved and the crystal length is 9–12 mm. The growth of the acicular crystals is promoted due to the addition of both TeO₂ and CaO.

For the powder compacts containing no Te, the growth of the acicular crystals is not observed. The obtained acicular crystals do not contain Te.

For both the powder compacts containing TeO₂ and the powder compacts containing both TeO₂ and CaO, the host phases have the (SrCa)₃TeO₆ crystal structure.

(3) Effect of Addition of (SrCa)₃TeO₆

An oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure was mixed with (SrCa)₃TeO₆ thereby preparing mixed powders having different compositions. The powders were calcined at 820° C. for 10 hours and then formed into powder compacts having a diameter φ of 15 mm and a thickness of 2 mm. The powder compacts were heat-treated at 870° C. for 100 hours in an atmosphere containing 10% of oxygen, thereby growing acicular crystals from the respective powder compacts. Table 3 shows the length of the acicular crystals, obtained by varying the content of (SrCa)₃TeO₆, having the Bi-2223 crystal structure.

TABLE 3 (SrCa)₃TeO₆ (mol) Length of Acicular Crystal (mm) 0.2 1~2 0.5 5~7 0.8 1~2

The acicular crystals having the Bi-2223 crystal structure are grown from the powder compacts containing one mol of the oxide Bi₂Sr₂Ca₂Cu₃O₁₀ and 0.2–0.8 mol of (SrCa)₃TeO₆. When the (SrCa)₃TeO₆ content is about 0.5 mol, the optimum effect can be achieved and the crystal length is 5–7 mm. The obtained acicular crystals do not contain Te.

(4) Effect of Temperature and Atmosphere of Heat Treatment

One mol of an oxide having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure was mixed with 0.5 mol of TeO₂ and 1.0 mol of CaO, thereby preparing a mixed powder. The powder was calcined at 820° C. for 10 hours and then formed into powder compacts having a diameter φ of 15 mm and a thickness of 2 mm. The powder compacts were heat-treated at different temperatures for 100 hours in atmospheres having different oxygen percentages, thereby growing acicular crystals from the respective powder compacts. The oxygen content was controlled by adding argon. Table 4 shows the length of the acicular crystals, obtained by varying the heat-treating temperature and oxygen content, having the Bi-2223 crystal structure.

TABLE 4 Heat Treating Oxygen Length of Acicular Temperature (° C.) Content (%) Crystal (mm) 840 10 1~2 860 10 9~12 880 10 6~8 890 10 2~3 860 5 4~6 860 20 7~9 860 100 2~4

The acicular crystals having the Bi-2223 crystal structure are grown from the powder compacts heat-treated at 840–890° C., in an atmosphere containing 10% of oxygen. When the temperature is 860° C., the crystal length is 9–12 mm. The acicular crystals are grown from the powder compacts treated in an atmosphere containing 5 to 100% of oxygen when the heat-treating temperature is 860° C. which is most effective in the growth. When the oxygen content is 10%, the crystal length is 9–12 mm. Thus, the optimum conditions of the heat treatment are as follows: the temperature is 860° C. and the oxygen content in the atmosphere is 10%.

The obtained acicular crystals were examined with an X-ray diffractometer, electron probe microanalyzer, and energy dispersive spectrometer. All the acicular crystals consisted of a single crystal of a Bi-2223 phase and did not contain the element Te that lowers the melting point of host phases.

The present invention is not limited to the above embodiments, and various modifications may be performed within a scope of the present invention. The invention is intended to cover such modifications.

As described above in detail, according to the present invention, the following advantages can be achieved.

-   (A) A method for producing a defect-free acicular crystal having a     Bi-2223 crystal structure is established, thereby obtaining     high-quality acicular crystals. -   (B) The achievement of the acicular crystal having the Bi-2223     crystal structure contributes to the development of superconducting     electronic elements that have been theoretically proposed but have     not been achieved. Thereby, switching elements operating at high     speed and high frequencies that have not been used can be achieved     and information technology is strongly stimulated.

INDUSTRIAL APPLICABILITY

The present invention relates to an oxide high-critical temperature superconductor acicular crystal that is substantially a perfect crystal and also relates to a method for producing the same. Such a crystal is fit for superconducting devices having a high operating frequency of several THz. 

1. A method for producing an oxide high-critical temperature superconductor acicular crystal, comprising a step of heat-treating a powder compact at 840 to 890° C. in an atmosphere containing 5 to 100% of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein the powder compact contains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of TeO₂.
 2. A method for producing an oxide high-critical temperature superconductor acicular crystal, comprising a step of heat-treating a powder compact at 840 to 890° C. in an atmosphere containing 5 to 100% of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein the powder compact contains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, 0.2 to 0.8 mol of TeO₂, and 0.1 to 2.0 mol of CaO.
 3. A method for producing an oxide high-critical temperature superconductor acicular crystal, comprising a step of heat-treating a powder compact at 840 to 890° C. in an atmosphere containing 5 to 100% of oxygen to grow an acicular crystal having a Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure, wherein the powder compact contains one mol of an oxide having the Bi₂Sr₂Ca₂Cu₃O₁₀ crystal structure and 0.2 to 0.8 mol of an oxide having a (SrCa)₃TeO₆ crystal structure. 